Plasma torches are conventionally used in the fields of cutting or welding of materials as well as in other related fields, such as plasma spraying, surfacing or marking.
A plasma torch usually consists of an electrode, made completely or partly of an emissive material, which electrode has a generally cylindrical or frustocylindrical elongate shape, of a nozzle placed coaxially with respect to the electrode and forming a diaphragm over the path of the plasma arc, of a circuit for internally cooling the torch, especially the electrode, and of one or more circuits for delivering a plasma gas into a chamber bounded, on the one hand, by the electrode and its support and, on the other hand, by the internal part of the nozzle and of its support.
In operation, the electrode is connected to one of the poles of a power supply, while the nozzle is connected to the other pole of the said power supply.
After ionizing part of the gas flux, which flows between the lower end of the electrode and a gas ejection channel made inside the nozzle, a primer electric arc is generated, thus creating an arc plasma column starting from the electrode and extending through the channel in the nozzle to the outside and as far as the material to be cut or welded, for example.
Depending on the type of use of the torch, the plasma jet is raised to a suitable power and maintained between the electrode, forming the cathode for example, and the nozzle, forming the anode, throughout the welding or cutting operation for example, or, depending on the case, the plasma jet is transferred to the workpiece, before the rise in power, by moving closer and by electrical switching, the workpiece then forming the anode and the nozzle then possibly being electrically disconnected.
Such plasma torches and their methods of operation have for example been described in documents EP-A-599,709, EP-A-573,330, U.S. Pat. No. 5,597,497, WO-A-96/23620, U.S. Pat. No. 5,451,739, EP-A-0,787,556, U.S. Pat. No. 5,416,296, U.S. Pat. No. 5,208,441, FR-A-2,669,847 and FR-A-2,113,144.
In certain cases, especially for technical or constructional reasons, the plasma gas is delivered into the chamber bounded by the electrode and the nozzle in an injection plane approximately perpendicular to the axis of the electrode.
Thus, the plasma gas may be injected into the chamber as a ring, centered on the axis of the electrode, via a continuous, circular slot.
In another situation, the plasma gas may be injected into the chamber via a ring-shaped component inside which there are gauged holes whose axes converge on and run into the electrode axis.
Finally, according to another embodiment, the plasma gas may be injected into the chamber via a ring pierced by gauged holes, as in the previous case, but this time these holes emerge tangentially to the bore of the chamber or, in an intermediate arrangement, between tangency to the bore and convergence of the axes of the holes on the electrode axis.
In the first two cases, the gas flux or the gas streams must undergo a sudden change of direction, which occurs when the gas flux is incident on the external periphery of the body of the electrode, or of its support, at an angle which is frequently about 90.degree..
However, it is known that such normal incidence causes turbulence, or even recirculation regions, within the gas flux and therefore disturbs the flow of the gas flux in the torch.
Therefore, in order to calm or control the flow, it is necessary to make or provide, downstream of the point of injection, a sufficient length of channel without any appreciable variation in the flow conditions so that the gas does not disturb, or disturbs as little as possible, the stability of the arc root attached to the end of the electrode during operation of the plasma torch.
In the latter case, the ring-shaped component pierced by holes, which emerge tangentially to the bore or, depending on the case, in an intermediate direction between tangency and convergence on the electrode axis, delivers the gas with turbulent flow.
However, the gas jets emanating from the injection holes and guided by the wall of the bore generally have a curved path lying approximately in a common plane.
Each gas jet therefore encounters, along its path, the next gas jet, when the order of delivery from the ring-shaped component pierced by holes is taken into consideration.
This therefore results, depending on the speed of flow of the gas jets and the exiguity of the chamber in the radial plane, in a disturbance of the gas flux which may cause stirring of all the gas jets, with the creation of considerable turbulence and an appreciable loss of vorticity or, as the case may be, an overlap of the gas jets in the vertical plane, this overlap being accompanied by extensive arc-root instability.
Consequently, whatever the actual embodiment, it is clear that disturbances to the flow of the gas flux will occur inside the plasma torch, causing appreciable instability of the plasma arc, which in turn has negative repercussions on the work carried out, which is of inferior quality.